The outer surface of an aircraft fuselage is typically prepared from composites, aluminum and/or steel. When prepared from aluminum or steel, the aircraft has a highly conductive path, like a Faraday cage, and the current can pass from the entry point of the strike across the skin to the exit point without greatly damaging the airplane. However, particularly with modern aircraft (and aircraft components), composite materials are being increasingly used to lower the weight of the aircraft. These materials often include carbon or graphite fibers, and the materials do not provide equivalent protection when compared with an all metal structure due to the lower electrical conductivity of the carbon or graphite.
Currently, lightning strike protection for composite materials, such as those used in aircraft construction, uses expanded metal screens (mesh) embedded in surface film attached on a composite surface to dissipate the energy incurred by a strike. The screens can be embedded in surface films or applied separately. U.S. Pat. No. 5,470,413 discusses a process for embedding a screen into a surface film. U.S. Pat. No. 5,417,385 discusses the fabrication of a structure with a lightning strike protective layer. Sometimes an extra layer of surface film is used to ensure a smooth surface for painting and to prevent microcracking. An extra fiberglass isolation ply can also be used if aluminum is used for the screen to prevent galvanic corrosion.
The aircraft and aerospace industry use certain composite structures to provide lightning strike protection. One such composite uses a 0.040 lb/ft2 (186 grams/m2 (gsm)) areal density copper screen, embedded in or placed on the surface of the component to be protected. A 0.030 lb/ft2 (140 gsm) areal density surface film (surface films are usually an adhesive or epoxy resin with fillers and modifiers) is usually cured with the composite structure and copper screen, resulting in a 0.070 lb/ft2 (326 gsm) areal density combination. However, due to concerns over the surface film microcracking, an additional layer of adhesive is sometimes added. The resulting areal density is around 0.10 lb/ft2 (466 gsm). Another composite structure employs a 0.016 lb/ft2 (74 gsm) areal density aluminum screen, which is placed on the surface of the component to be protected. A 0.05 lb/ft2 (232 gsm) surface film is placed over the aluminum screen. If the material of the component is a carbon composite, the aluminum screen also requires a fiberglass isolation layer, typically 0.091 lb/ft2 (423 gsm). The isolation layer is provided to prevent galvanic corrosion with the underlying carbon composite, and it also assists in lightning strike protection since aluminum is not as conductive as copper. The total areal density of this composite structure is 0.157 lb/ft2 (730 gsm).
There is an interest in reducing the density of lightning strike protection materials. Research in this area has focused on using metal particles, foils, and/or screens. Examples include copper powder, applied in the form of a paint, and copper screen, embedded in an epoxy or other polymer coating layer. In some embodiments, these materials provided adequate protection against lightning strikes. However, at least in part due to the difference in the coefficients of thermal expansion (CTE's) between the metal particles/screens and polymers, these materials may microcrack during the thermal cycling conditions experienced by an airplane under flight conditions. That is, at high altitudes, the airplane (and lightning strike protection materials) experiences relatively low temperatures, and on the ground, the airplane is exposed to relatively higher temperatures. The change in temperatures can be extreme, and can result in microcracking if the conductive material and the polymer used in the lightning strike protection materials have significantly different CTE's. This microcracking can lead to moisture or chemical ingress into the composite structure resulting in the potential for a reduction in mechanical properties of the structure.
It would be advantageous to provide lightning strike protection materials with a density lower than currently used lightning strike protection materials, and which is capable of surviving the thermal cycling to which an airplane is exposed under conditions of use. The present invention provides such lightning strike protection materials, aircraft and aircraft components including these materials, and methods for preparing these materials. For purposes of this invention, the term “aircraft components” is intended to include the various parts of an aircraft, including without limitation, the fuselage portion of the aircraft, the aircraft's various control surfaces (such as flaps, slats, tail, etc.), and the aircraft propulsion system and its various components (the engine, nacelle, pylori, etc.).